The invention relates to a background subtractor circuit for use as part of an overall system to measure radioactivity in a test tube sampling apparatus or the like. It is known in the art to sample various human or animal fluids, including blood, and to analyze such fluids by placing the fluids in a test tube and associating the same with radioactive material. By known techniques, information regarding such bodily fluids can be developed by measuring the gamma ray emission from the material in the test tube.
Detectors or sensors that pick up, or sense, the gamma ray emissions from such test tubes are well known in the art, and the most advanced are typically automatic which operate on a plurality of test tubes in a given procedure. For a more detailed discussion of the procedure noted above, and for a full disclosure of an automatic sample changer, see U.S. Pat. No. 4,001,584, issued to the present assignee on Jan. 4, 1977.
In the known systems, the gamma rays detected are converted to voltage pulses or spikes, and are then fed to a pulse height analyzer circuit. The pulse height analyzer circuit filters out unwanted pulses by passing pulses that are only within a certain minimum and maximum height. Although the pulse height analyzer filters out many of the unwanted pulses, it has been found, in practice, that not all of the unwanted pulses are removed. Frequently, a number of unwanted so-called background pulses still exist, largely caused by background or environmental noise. It is important for the operator of the system to be aware of the unwanted pulses that are caused by the background or environmental noise, for, otherwise, inaccuracies in readings are likely to result.
It is an object of the present invention to provide an apparatus whereby an operator can manually select, by means of thumbwheel switches or other conventional switches known in the art, the number of pulses that are caused by noise or environmental factors, and by thus setting the switch to the number of pulses of background noise per minute, the pulses are automatically subtracted from the incoming pulse rate from the pulse height analyzer, thus providing an output reading that is devoid of background pulses.
In order for the operator to determine the amount of background pulses to be removed, the apparatus is set up in the particular environment where it is to be used and the operator takes a reading for a given period of time, one minute for example, without measuring any radioactivity from the test tube. The number of pulses obtained in one minute represent the background noise caused by environmental factors, such as naturally occurring gamma ray emitting Isotopes, photomultiplier noise and cosmic rays. A reading is obtained for a sufficient length of time and the operator then calculates the amount of background noise, or pulses, per minute. This reading of background noise pulses per minute is then set by the operator into a background subtractor circuit that constitutes the present invention. In operation, the background pulses are automatically subtracted from the incoming pulse rate from the pulse height analyzer.
Generally, the pulse output from the pulse height analyzer, representing the gamma ray emissions plus background noise, are analyzed in two distinct modes. One mode, called the preset time mode, takes a reading for a preset period of time and counts the number of pulses in that time frame. The other mode, called the preset count mode, measures the amount of time that it takes to receive a fixed number of pulses. Generally, when measuring radioactivity that has a fairly low count rate, it is desirable to use the preset time mode wherein the number of pulses in a given period of time are measured. This is largely for reasons of economy since if the count rate is very low, it would take a substantial amount of time to receive a given number of pulses. It is thus a further object of this invention to provide a background subtractor that is capable of being used in either the preset time or preset count mode.
When using the system for rather low count rates in the preset time mode, at the end of the measured time interval only a small number of detected pulses may be sensed. In such a case, it is necessary that all background pulses in that time interval be subtracted to provide an accurate reading. Thus, it is an object of the background subtractor of the present invention to include a means whereby the initial pulses of the incoming pulse train are delayed by means of a shift register, and later cleared at the end of the preset time interval in order to assure that all background noise pulses are removed.
These and other objects of the present invention, as well as many of the attendant advantages thereof, will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings.